|The external hard drive MUST be plugged into a USB port that can support it's requirements. The USB port MUST be able to supply 500ma of current, and on top of that, external hard drives (and external optical drives) do not work in all possible USB ports.|
If the external drive IS plugged into a USB port that can support it's requirements....
- assuming you have NOT dropped the external drive or have damaged it otherwise, there is probably NOTHING wrong with the external enclosure's circuits or the hard drive inside the external enclosure. In that case, the hard drive inside the external enclosure will show up in Disk Management in Windows, even if data on the drive is damaged such that no drive letter(s) for the external drive show(s) up in Computer or Windows Explorer.
If the data on the hard drive IS damaged, it MAY say RAW (= unallocated) rather than Healthy in the information in the box (one partition) or boxes (more than one partition) for the drive on the right.
- the most common reason people can no longer access data that they know is still on the external drive, which may result in the partition(s) on the drive NOT showing up in Computer or Windows Explorer, is the drive was unplugged while Windows was running WITHOUT the user using Safely Remove Hardware to STOP accessing the partition(s) on the drive BEFORE unplugging it, and data on the drive was damaged because of that. Usually if you use certain data recovery programs that can repair the data damage, you will be able to access your data, or most of it, that you could access previously.
If (the) partition(s) on the drive was (were) not showing up in Computer or Windows Explorer, it's the partition table data that was damaged and needs to be repaired.
Troubleshooting USB device problems including for flash drives, external drives, memory cards.
See Response 1: